Guided waves (GW) allow fast inspection of a large area and hence have received great interest from the structural health monitoring (SHM) community. Fiber Bragg grating (FBG) sensors offer several advantages but their use has been limited for the GW sensing due to their limited sensitivity. FBG sensors in the edge-filtering configuration have overcome the issue with sensitivity and there is a renewed interest in their use. The FBG sensors have directional sensitivity and are passive in nature. This makes it difficult to apply the existing signal processing and damage detection techniques such as tomography, phased array approach etc. As a result, there is a need to develop novel damage detection techniques which take into consideration the specific limitations of the use of FBG sensors. This paper applies the ellipse and hyperbola based techniques for a FBG sensor based network. The paper identifies the specific challenges and applies solutions to overcome those challenges. A comparative assessment of the two methods is presented with the help of experimental data.
We experimentally demonstrate spectral manipulation of heralded single photons by electro-optic temporal phase modulation employing complex, Fresnel-like wide-band electronic waveforms. We show spectral compression of classical telecom light enhancing its maximal intensity by over 80, by compressing its full-width at half maximum (FWHM) bandwidth from 0.856 nm down to 3.1 pm (383 MHz), thus achieving a compression factor of over 270. We show a compression gain by increasing the aperture of a Fresnel time lens (temporal waveform duration). We demonstrate a similar compression on a single-photon level from ~1.5 nm (~190 GHz) down to the sub-2GHz regime. Our findings should significantly improve the performance of future quantum information processors based on hybrid quantum networks.
We present a scheme for generation and characterization of entangled spatial qubits based on type-II spontaneous parametric down-conversion (SPDC) in a periodically poled titanyl phosphate (PPKTP) multimode nonlinear waveguide . Our scheme exploits intermodal dispersion which has been hitherto successfully employed to produce spatially pure SPDC photon pairs from a multimode waveguide without spatial filtering . Production of discrete
entanglement relies on driving simultaneously two SPDC processes that involve different combinations of transverse spatial modes for which phase matching bandwidths significantly overlap. We propose a procedure for experimental identification of the spatial qubit subspace based on a scan of the spatial Wigner function via the displaced parity measurement using an inverting Sagnac interferometer and photon counting. We numerically verified the robustness
of the mode reconstruction procedure against experimental imperfections. We also propose an experimental method for detecting spatial entanglement in the position-wave vector phase space. Numerical simulations indicate that waveguide parameters required for experimental demonstrations are compatible with current manufacturing capabilities. Using simulated mode profiles we calculate the maximum attainable Clauser-Horne- Shimony-Holt combination value reaching 2.12, which clearly violates the classical limit and confirms the feasibility of observing non-classical features of the generated state.
 M. Jachura et al. Physical Review A, 95, 032322 (2017).
 M. Jachura, M. Karpiński, C. Radzewicz, K. Banaszek, Optics Express, 22, 8624-8632 (2014).
Spectral-temporal manipulation of optical pulses has enabled numerous developments within a broad range of research topics, ranging from fundamental science to practical applications. Within quantum optics spectral-temporal degree of freedom of light offers a promising platform for integrated photonic quantum information processing. An important challenge in experimentally realizing spectral-temporal manipulation of quantum states of light is the need for highly efficient manipulation tools. In this context the intrinsically deterministic electro-optic methods show great promise for quantum applications.
We experimentally demonstrate application of electro-optic platform for spectral-temporal manipulation of ultrashort pulsed quantum light. Using techniques analogous to serrodyne frequency shifting we show active spectral translation of few-picosecond single photon pulses by up to 0.5 THz. By employing an approach based on an electro-optic time lens we demonstrate up to 6-fold spectral compression of heralded single photon pulses with efficiency that enables us to significantly increase single photon flux through a narrow bandpass filter.
We realize the required temporal phase manipulation by driving a lithium niobate waveguided electrooptic modulator with 33 dBm sinusoidal RF field at the frequency of either 10 GHz or 40 GHz. We use a phase lock loop to temporally lock the RF field to the 80 MHz repetition rate of approximately 1 ps long optical pulses. Heralded single photon wavepackets are generated by means of spontaneous parametric down-conversion in potassium dihydrogen phosphate (KDP) crystal, which enables preparation of spectrally pure single photon wavepackets without the need for spectral filtering. Spectral shifting is achieved by locking single-photon pulses to the linear slope of sinusoidal 40 GHz RF phase modulation. We verify the spectral shift by performing spectrally resolved heralded single photon counting, using frequency-to-time conversion by means of a highly dispersive chirped fiber Bragg grating. We verify the non-classicality of spectrally shifted single photons by measuring high-visibility Hong-Ou-Mandel interference using a reference single photon pulse.
Spectral compression is based on the time lens principle, which requires locking optical pulses to approximately quadratic region of sinusoidal phase modulation. We utilize both 10 GHz and 40 GHz RF driving frequencies. Bandwidth compression is achieved by chirping the single photon pulse using an appropriate length of single-mode fibre and subsequently subjecting it to the action of the time lens. We verify spectral compression directly using the aforementioned spectrally-resolved heralded single photon counting method. We achieve 3-fold spectral compression of 2 nm bandwidth single photon pulses using 40 GHZ modulation frequency, and 6-fold spectral compression of 0.9 nm bandwidth single photon pulses using 10 GHz modulation frequency. Overall transmission of our set-up exceeding 30% enables practical usability of our spectral compression method which we demonstrate experimentally by showing an increased photon flux through a narrowband filter.
Our results present an important contribution towards implementing quantum information processing in the spectral-temporal degree of freedom of a photon. In the context of quantum networks they present an enabling tool towards efficient photonic interfacing of different quantum information processing platforms.
We present experimental realization of type-II spontaneous parametric down-conversion in a periodically poled
potassium titanyl phosphate (KTiOPO4) nonlinear waveguide. We demonstrate that by careful exploitation of
intermodal dispersion in the waveguide it is feasible to produce photon pairs in well defined transverse modes
without any additional spatial filtering at the output. Spatial characteristics is verified by measurements of
the M2 beam quality factors. We also prepared a postselected polarization-entangled two-photon state shown
to violate Bell’s inequality. Similar techniques based on intermodal dispersion can be used to generate spatial
entanglement and hyperentanglement.